NICER observation list: Target ID 459401

Understanding the wind structure, i.e., clumpiness of winds in high-mass X-ray binaries is important to learn more about binary evolution and understand a crucial part of the accretion process. The clumps strongly influence the observed X-rays, in particular through variable absorption. Here we ask to monitor 4U 1538-522 and its mass donor QV Nor for a complete binary orbit (3.7d) with 1ks snapshots every 90min. This will allow us to build an absorption and wind profile over the orbit and measure the structures close to the photosphere of QV Nor. NICER's flexibility is ideally suited for this observation campaign.